Monitor MZ25C Circulation Question

OttoPilot

Hello.



I have a Monitor MZ25C boiler that trips out on high water temperature.  I used an IR thermometer at the top on the heat exchanger casting to verify the 198°F  trip temperature.  The installation is about 6 years old.



It acts as if it has low water flow because the heat exchanger cools down very slowly after the trip; whereas, before this problem the heat exchanger cooled within 30 seconds of flame off (pump still running).



The pressure gauge on the boiler now shows a 7 psi increase whenever the pump runs   Previously there was no difference in pressure with the state of the pump.



Water will flow to drain in a good stream from the return side of the boiler (supply

side shut off).  Water will flow at a slightly lower rate backwards through the boiler (return side shut off).  Water starts to blow back from the hub drain grating if I open the ½” drain valve more than 75%, so I’m not sure of the ultimate drain flow rate in either direction.



I am supplying cast iron radiators through piping sized for a gravity system, so I ran

the pump on 1st (lowest) speed until I had the trip problem last week.  Changing to 3rd speed had little effect on the problem.



I’m not getting through to Bugsy at Monitor to help with the diagnosis.



Is there anything else I should check to narrow down the problem?



Thanks.

pipeking

YOUR ON THE RIGHT PATH

 i sounds like a there is a clog somewhere in thet arrea. was there a strainer installed on the system that is a very important accesserory, aspecially with a radiator retrofit.

OttoPilot

Strainer

There is a 2" strainer in the return line.  I thought perhaps it was plugged, but there was good forward flow to drain through the strainer.

Alan (California Radiant) Forbes

Strainer

There is also a strainer in the boiler that you should check.  As far as I remember, it's in the lower right hand corner and has a removable cap with a chain on it.  I've had them clog on me before.

Kniggit

little more

There is a little more to it than that, but I can post some pictures of the exact location on mine if you need them.



K

OttoPilot

Strainer

Thank you for that suggestion, That possibility is consistent with all the facts.  I had always wondered what the brass arrangement on the lower right was.



I have an early job, so I'll leave the $200 of electric heaters I bought yesterday to do their job until I get home at 4 today.  I'll check then and post the result.

OttoPilot

Strainer

I drained the boiler, removed the large union nut, and found...

A clean strainer!



Are there other possibilities?

CMadatMe

When Was the Last Time

The heat exchanger was serviced? Did you check the pump for any build up ? Any chance of a flue blockage?

OttoPilot

Cleaning and Pump Inspection

I washed the fire side of the heat exchanger 1 year ago, and there are no faults to indicate restricted intake air flow or excess pressure on the flue gas.  The overtemp fault is on the water jacket of the heat exchanger rather than on the flue gas.  The fault appears on to be connected to slow water circulation, based on how slowly the heat exchanger cools off after the gas trips off.

I'm not sure if you meant to clean the water side of the heat exchanger and I'm not sure how I would do that.  When I let water flow through the heat exchanger in reverse flow to drain, it flowed well and there was no sediment, consistent with the state of the strainer I inspected.  Based on the water-side fault and lack of flue gas overtemp fault, I thought the heat exchanger was clean enough to do its job.

I will remove the pump from the line to visually check it.

The first overtemp trip was on January 8, the second on January 14, and now it won't run for over 1 minute without tripping.  I'm saying the problem certainly came on quickly after 6 years of trouble-free operation.

OttoPilot

Heat Exchanger Blockage

I reconfigured my piping so I could use city water to flow through the boiler.  At 45 psi inlet pressure, the flow rate initially was about 5 gpm.  Some pieces of  very soft debris came out and the flow rate gradually decreased to about half that.  The debris broke down to what looked like fine black rust powder when you touched it.

This is consistent with the history of this problem--it has gotten worse over time as if the blockage is getting worse.

Judging from the inlet versus outlet temperature and the heat input rate, the normal flow rate though the entire system was about 5 gpm.  The pump has an output of 5 ft of water at that flow rate.

So there is now less flow at 45 psi than there was at 5 ft of water.

Any suggestions for clearing the heat exchanger?

bob_46

Magnetite

Try sticking a magnet in that black powder and see if it is magnetic . I would put TSP in the system and let it circulate a few days . Drain it and flush it well . Put a strong magnet in your 2" strainer .